Numerical design of RN(n)(nu) symmetry-based RF pulse schemes for recoupling and decoupling of nuclear spin interactions at high MAS frequencies.

An approach for the efficient implementation of RN(n)(nu) symmetry-based pulse schemes that are often employed for recoupling and decoupling of nuclear spin interactions in biological solid state NMR investigations is demonstrated at high magic-angle spinning frequencies. RF pulse sequences belonging to the RN(n)(nu) symmetry involve the repeated application of the pulse sandwich {R(phi)R(-phi)}, corresponding to a propagator U(RF) = exp(-i4phiI(z)), where phi = pinu/N and R is typically a pulse that rotates the nuclear spins through 180 degrees about the x-axis. In this study, broadband, phase-modulated 180 degrees pulses of constant amplitude were employed as the initial 'R' element and the phase-modulation profile of this 'R' element was numerically optimised for generating RN(n)(nu) symmetry-based pulse schemes with satisfactory magnetisation transfer characteristics. At representative MAS frequencies, RF pulse sequences were implemented for achieving 13C-13C double-quantum dipolar recoupling and through bond scalar coupling mediated chemical shift correlation and evaluated via numerical simulations and experimental measurements. The results from these investigations are presented here.